This invention relates to a semiconductor laser array and its manufacturing method, optical integrated unit, optical pickup and optical disk driving apparatus. More specifically, the invention relates to a semiconductor laser array for a short wavelength band and its manufacturing method; compact high-performance optical integrated unit, optical pickup and optical disk driving apparatus suitable for use in a compatible optical disk system such as DVD system ensuring compatibility with CD or CD-R, for example, using such a laser.
Optical disk systems are under wide practical use because they are compact but capable of recording a large amount of data. DVD (digital versatile disc) systems, in particular, are under rapid development toward practical use as major systems such as next-generation movies, ROMs and RAMs. On the other hand, CD (compact disc) systems or CD-R (compact disc-recordable) systems have been widely diffused for years, and DVD systems are desired to be compatible with CD systems. That is, DVD systems are required to be capable of reading and writing data on and from CDs or CD-Rs.
In these optical disc systems, an optical pickup using a semiconductor laser (LD) is used to read and write information on and from a disc.
FIG. 26 is an explanatory view showing a typical construction proposed as an optical pickup for conventional DVD systems. The optical pickup shown here has a compatibility with CDs, and includes an optical integrated unit 101 for DVDs and another optical unit 102 for CDs and CD-Rs.
Laser light of the wavelength 650 nm released from the DVD-compatible optical integrated unit 101 passes through a dichroic prism 103, then through a collective lens 104, re-orienting mirror 105, wavelength selecting filter 106 and objective lens 107, and reaches an optical disc 109. On the other hand, laser light of the wavelength 780 nm released from the CD-compatible optical integrated unit 102 is first reflected by the prism 103, then travels the same path as the laser light of 650 nm for DVDs, and reaches CD or CD-R 108.
Return light from the disc travels the optical path in the opposite direction, and reaches the DVD-compatible optical integrated unit 101 or CD-compatible optical integrated unit 102.
In general, since the spot size by the objective lens 107 is slightly different between CD 108 DVD-ROM disk 109, the effective NA (numerical aperture) is changed by using the wavelength selecting filter 106, for example.
Next explained is a conventional optical integrated unit used with the optical pickup shown above.
FIG. 27 is a perspective view schematically showing construction of typical conventional optical integrated units. Optical integrated units 101, 102 have a stem 138 and a heat sink 141 mounted thereon. Adequately provided on the stem 138 are leads 109 for predetermined electrical connection. The heat sink 141 is made of a material having a good heat conductivity, such as copper, and a LD chip 135 and a detecting PD (photodiode) 136 are provided thereon. Monitoring PD 137 is provided behind the LD chip 135 to feedback-control the LD optical output.
Above those elements, a hologram element, not shown, is provided. A stem encapsulating cap is omitted from illustration in FIG. 27.
Light released from the LD chip 135 in the direction shown with an arrow in FIG. 27 reaches the disk through the path explained with reference to FIG. 26. Return light from the disk is diffracted by the hologram, and enters the error detecting PD 136 as shown with an arrow in FIG. 27. PD is divided into some regions for detecting the optical focus and tracking errors on the disk. For example, PD can be designed to equalize quantities of incident light among respective divisional regions when the disk is positioned at a focal point. If it moves from the focal point, then a difference is produced in quantities of incident light among the divisional regions. Therefore, by detecting it as a current difference, it is fed back via a mechanical servo mechanism, not shown, to return the disk to the focal point. Detection of errors in radial directions also follows the same process.
The conventional optical pickup, however, involved problems, namely, complicated construction, difficulty in reducing its size and weight, and the need for complicated assemblage. These problems are discussed below in greater detail.
In the conventional optical pickup shown in FIG. 26, beams of light from two different optical integrated units 101, 102 must be synthesized into a single optical axis because the angular difference of light from LD relative to the optical axis of the pickup must be maintained minimum. For this purpose, it required optical parts like the dichroic prism 103, and this resulted in complicating the construction, increasing the size, complicating the assembling process and increasing the cost.
Moreover, the conventional optical pickup is subject to degradation of the ratio of acceptable products through the assembling process because of the need for the process of adjusting optical axes of two different optical integrated units 101, 102 used therein. That is, also for the positional accuracy (X, Y, xcex8) of return light from the disk (diffracted light from the hologram), there is a strict requirement. Especially in DVD, the positional accuracy is desired to be within xc2x15 xcexcm, xc2x10.50xc2x0 between LD and PD. Even if the light from LD is within a desired accuracy, expected characteristics are not obtained unless relative positions of PD and LD in each optical integrated unit are held within the above-mentioned acceptable range. That is, since bifurcated optical integrated units are used, relative positional accuracy must be sufficiently high between two LDs and two PDs. The increased number of steps for adjustment required to realize it and degradation in the ratio of acceptable products of the pickup through assemblage are serious problems.
Furthermore, the use of two divisional optical integrated units limits miniaturization of the entirety. Especially when DVD systems are mounted in portable personal computers whose demand is expected to greatly grow in the future, reduction of the size and the weight is indispensable. However, it has been significantly difficult to reduce the size and the weight with the conventional structure as shown in FIG. 26.
It is therefore an object of the invention to provide a high-performance semiconductor laser array of a multi-wavelength type and its manufacturing method, and to provide a compact, high-performance optical integrated unit, optical pickup and optical disk driving apparatus which can be realized by using such semiconductor lasers.
According to the invention, there is provided a semiconductor laser array comprising: a GaAs substrate; a first laser element portion provided on said substrate to release laser light of a first wavelength; and a second laser element portion provided on said substrate to release laser light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of the first wavelength, said first laser element portion including a first cladding layer, an active layer formed by epitaxially growing a first semiconductor material on said first cladding layer, a second cladding layer formed on said active layer and a current-blocking layer to confine an electrical current injected into said first laser element portion, said second laser element portion including a first cladding layer, an active layer formed by epitaxially growing a second semiconductor material on said first cladding layer, a second cladding layer formed on said active layer and a current-blocking layer to confine an electrical current injected into said second laser element portion, and said current-blocking layer of said first laser element portion and said current-blocking layer of said second laser element portion are made of same semiconductor material.
According to the invention, there is further provided a semiconductor laser array comprising:
a GaAs substrate;
a first laser element portion provided on said substrate to release laser light of a first wavelength; and
a second laser element portion provided on said substrate to release laser light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of the first wavelength,
said first laser element portion including a first cladding layer made of InGaAlP, an active layer formed on said first cladding layer, a second cladding layer formed on said active layer and made of InGaAlP, a stripe-shaped intermediate layer formed on said second cladding layer and made of a semiconductor material having a smaller band gap than said second cladding layer, and top layer formed to cover said second cladding layer and said intermediate layer and made of a semiconductor material having a smaller band gap than said intermediate layer.
said second laser element portion including a first cladding layer made of InGaAlP, an active layer formed on said first cladding layer, a second cladding layer formed on said active layer and made of InGaAlP, a stripe-shaped intermediate layer formed on said second cladding layer and made of a semiconductor material having a smaller band gap than said second cladding layer, and top layer formed to cover said second cladding layer and said intermediate layer and made of a semiconductor material having a smaller band gap than said intermediate layer.
According to the invention, there is further provided a manufacturing method of a semiconductor laser array having a GaAs substrate, a first laser element portion provided on said substrate to release laser light of a first wavelength, and a second laser element portion provided on said substrate to release laser light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of the first wavelength, comprising the steps of:
making a double-heterostructure of a first cladding layer, an active layer and a second cladding layer forming said first laser element portion in a location on a major surface of said GaAs substrate;
making a double-heterostructure of a first cladding layer, an active layer and a second cladding layer forming said second laser element portion on another location on said major surface of said GaAs substrate;
selectively etching said second cladding layer of said first laser element portion and said second cladding layer of said second laser element portion simultaneously to form stripes extending along laser cavity lengthwise directions, respectively; and
making an element separation groove between said first laser element portion and said second laser element portion to block an electric current therebetween.
According to the invention, there is further provided a manufacturing method of a semiconductor laser array having a GaAs substrate, a first laser element portion provided on said substrate to release laser light of a first wavelength, and a second laser element portion provided on said substrate to release laser light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of the first wavelength, comprising the steps of:
making a double-heterostructure of a first cladding layer, an active layer and a second cladding layer forming said first laser element portion in a location on a major surface of said GaAs substrate;
making a double-heterostructure of a first cladding layer, an active layer and a second cladding layer forming said second laser element portion on another location on said major surface of said GaAs substrate;
making intermediate layers having a smaller band gap than said second cladding layers on said second cladding layers of said first and second laser element portions;
selectively etching said intermediate layers of said first and second laser element portions simultaneously to form stripes extending along laser cavity lengthwise directions, respectively;
making top layers having a smaller band gap than said intermediate layers over said first and second laser element portions; and
making an element separation groove between said first laser element portion and said second laser element portion to block an electric current therebetween.
When the first wavelength is longer than the second wavelength, the step of making the double-heterostructure of the first laser element portion preferably precede the step of making the double-heterostructure of the second laser element portion.
The second cladding layer of the first laser element portion may have a p-type conduction type and its p-type carrier density is preferably not larger than 8xc3x971017 cmxe2x88x923.
The step of making the stripes may include a step of selective etching terminated at etching stop layers provided in the first laser element portion and the second laser element portion, respectively.
The first wavelength may range about 780 nm as its center, and the second wavelength may range about one of 635 nm or 650 nm as its center.
According to the invention, there is further provided an optical integrated unit comprising: a integrated laser array including a first laser element portion and second laser element portion integrated on a common substrate, said first laser element portion releasing laser light of a first wavelength, said second laser element portion releasing light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of said first wavelength; and detector means for detecting first return light which is part of the laser light of said first wavelength reflected back in the exterior and second return light which is part of the laser light of said second wavelength reflected back in the exterior.
According to the invention, there is further provided an optical integrated unit comprising: a first laser element portion releasing laser light of a first wavelength; a second laser element portion releasing light of a second wavelength different from the first wavelength in a direction substantially parallel to the laser light of the first wavelength; and
holographic optical element for diffracting first return light which is part of the laser light of the first wavelength reflected back in the exterior by a first diffraction angle and diffracting second return light which is part of the laser light of the second wavelength reflected back in the exterior by a second diffraction angle different from the first diffraction angle; and detector means for detecting the first return light and the second return light diffracted by the holographic optical element at a substantially common detecting position.
According to the invention, there is further provided an optical integrated unit comprising: a first laser element portion releasing laser light of a first wavelength; a second laser element portion releasing light of a second wavelength different from the first wavelength in a direction substantially parallel to the laser light of the first wavelength; first detector means for detecting first return light which is part of the laser light of the first wavelength reflected back in the exterior; and second detector means for detecting second return light which is part of the laser light of the second wavelength reflected back in the exterior.
According to the invention, there is further provided an optical integrated unit comprising: a first laser element portion releasing laser light of a first wavelength; a second laser element portion releasing light of a second wavelength different from the first wavelength in a direction substantially parallel to the laser light of the first wavelength; and
holographic optical element for diffracting first return light which is part of the laser light of the first wavelength reflected back in the exterior by a first diffraction angle and diffracting second return light which is part of the laser light of the second wavelength reflected back in the exterior by a second diffraction angle different from the first diffraction angle; first detector means for detecting the first return light diffracted by the holographic optical element; and second detector means for detecting the second return light diffracted by the holographic optical element.
The first detector means and the second detector means may be any of a plurality of photo diodes integrated on a common substrate.
Any of the optical integrated elements summarized above may further comprise a third laser element portion releasing laser light of a third wavelength.
The first laser element portion and the second laser element portion may form a laser array integrated on a common substrate.
The laser array may be a semiconductor laser array recited in one of claims 1 to 5.
The optical integrated unit summarized above may further comprise a silicon substrate having at least one step portion on a major surface thereof, the laser array being mounted on a lower part of the major surface at one side of the step portion of the silicon substrate to release the laser light of the first wavelength and the laser light of the second wavelength toward a side surface of the step portion, the side surface of the step portion including a reflector portion for reflecting the laser light of the first wavelength and the laser light of the second wavelength approximately perpendicularly upward relative to the major surface of the substrate.
The holographic optical element may have a hologram element.
The first wavelength may range about 780 nm as its center, and the second wavelength may range about one of 635 nm, 650 nm and 685 nm as its center.
According to the invention, there is further provided an optical pickup comprising: an optical integrated unit including a integrated laser array and detector means, said integrated laser array including a first laser element portion and second laser element portion integrated on a common substrate, said first laser element portion releasing laser light of a first wavelength, said second laser element portion releasing light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of said first wavelength, said detector means detecting first return light which is part of the laser light of said first wavelength reflected back in the exterior and second return light which is part of the laser light of said second wavelength reflected back in the exterior; and holographic optical element for diffracting first return light which is part of the laser light of said first wavelength reflected back in the exterior by a first diffraction angle and diffracting second return light which is part of the laser light of said second wavelength reflected back in the exterior by a second diffraction angle different from said first diffraction angle.
According to the invention, there is further provided an optical pickup comprising: an optical integrated unit including a integrated laser array and detector means, said integrated laser array including a first laser element portion and second laser element portion integrated on a common substrate, said first laser element portion releasing laser light of a first wavelength, said second laser element portion releasing light of a second wavelength different from said first wavelength in a direction substantially parallel to the laser light of said first wavelength, said detector means detecting first return light which is part of the laser light of said first wavelength reflected back in the exterior and second return light which is part of the laser light of said second wavelength reflected back in the exterior; and an optical system for converging laser light of a first wavelength released from said optical integrated unit or laser light of a second wavelength and irradiating it onto an optical disk, and for guiding light reflected back from said optical disk to said optical integrated unit.
According to the invention, there is further provided an optical disk driving apparatus comprising one of optical pickups summarized above.